Optical fibers intended for transmission of optical signals are extremely susceptible to microbending loss. This type loss traces to small scale axial undulations of the fiber which cause coupling of the light into lossy modes. Coatings applied over the bare glass fiber are one means of cushioning the fiber from physical sources which impart the axial undulations to the fiber. One such coating system is described in the application of D. Gloge, Ser. No. 439,029, filed July 30, 1974 now U.S. Pat. No. 4,000,936, and assigned to applicants' assignee. The coating can serve also to protect the fiber from abrasion, and to hold in place protective materials that might be previously placed on the fiber.
The splicing of two optical fibers requires a greater sophistication than splicing of two copper conductors, in that the optical fibers to be joined must be abutted or closely spaced in end-to-end relation and in axial alignment. This requirement strongly favors optical fiber cables configured internally in ways that facilitate ease of splicing. To this end, a flat ribbor structure of fibers with predetermined center-to-center spacing is a desirable and often preferred arrangement, both for protecting the fibers to some extent from microbending loss and for prepositioning them preparatory to splicing. One such optical fiber ribbon structure is shown for example in the patent application Ser. No. 403,380 of C. M. Miller, now U.S. Pat. No. 3,984,172, filed Oct. 23, 1973. Ribbon structures also are a building block for a multilayer optical fiber ribbon cable. Desirably, in such a cable the fiber axes are arranged cross-sectionally in an X-Y matrix of uniform center-to-center spacing.
Microbending loss can be brought on by surprisingly slight perturbations; and consequently very close and careful control over the characteristics of a protective coating must be maintained. However, the nature of the desired control has heretofore been difficult to identify. Moreover, the need for this control over fiber coating characteristics has further complicated the creation of a workable optical fiber ribbon structure.